I just looted the Copper Key from a goblin interrogator in the jail in Droga. People are correct that you will need shrink to move through the locked gates.
Also, while none of the goblins seemed to see invis, either the spider at the bottom of the drop into the jail saw through it, or there was a trap that caused it to drop.
Edited, Jul 29th 2018 1:26pm by DyllwinDW